Low-Pressure Retention Locking with the 789A GC Application HPI Authors Courtney Milner and Russell Kinghorn BST International 41 Greenaway Street Bulleen, VIC 315 Australia Matthew S. Klee Agilent Technologies, Inc. 285 Centerville Road Wilmington, DE 1988 USA Abstract Retention time locking was introduced over a decade ago with the Agilent 689 gas chromatograph. The next generation of GC from Agilent the 789A has enhanced and extended the functionality previously available, including a new electronic pneumatic control system capable of pressure control to the third decimal place. This application demonstrates the ability of the EPC system to be used for retention time locking at low pressures, in this case with a 32-µm column on a 5975 GC-MS. Introduction The introduction of retention time locking (RTL) with the 689 GC gave users a new way of improving productivity by eliminating the need to constantly update compound retention time data whenever a column was trimmed or replaced. It also allowed the same methods to be run on multiple systems with the same retention times. The introduction of emethods further enhanced the portability of these methods. Most RTL-based methods were established on GC/MS systems, where a typical head pressure for a 3 m.25 µm id column is > 1 psi. With setability to two decimal places, four-digit pressure setpoints for such columns (for example, 11.54 psig) result in excellent inter-instrument and intrainstrument RTL precision. Amongst the many optimization tasks in GC method development is deciding on the best column dimension to select. This can realistically only be determined with full knowledge of the sample characteristics and analysis goals (that is, components of interest, complexity, detection limits required, and the matrix of the sample). Larger diameter columns have the advantages of ruggedness and sample capacity over smaller dimension columns. The larger the diameter of the column, the less pressure is needed to establish optimal flows. However, for the most precise RTL, one needs the ability to set pressure very precisely. With two-decimal-place precision at low pressures (for example, 1.28 psig), locking a system to a target retention time is less precise for column dimensions, such as.32-µm columns. The 789 GC s fifth generation EPC provides excellent low-pressure control, and with third-decimalplace control of the pressure, providing the precision demanded for RTL at low pressures. This application explores the suitability of the 789 for RTL at low pressures with a 32-µm column and uses a translation of the method described in Agilent Technologies publication 5989-6569EN, Reliable transfer of existing Agilent 689/5973GC/MSD methods to the new 789/5975 GC/MSD.
Experimental The method used for this example was translated using the Agilent Method Translator software to convert to a 25-µm id column method to a 32-µm method. A series of standards was run from 1 ppb to 5 ppm to illustrate the performance of the method, followed by trimming approximately 45 cm off the column and relocking the method. Table 1. Method Conditions Column HP-5 25 m.32 µm.52 µm p/n 1991J-112 Carrier gas Helium, constant pressure mode nominal 3.761 psi RTL peak Fluoranthene at 11.112 min Split/splitless inlet 3 C, pulsed splitless 7 psi for.3 in, 5 ml/min purge at.75 min Oven 55 C (1.1 min) to 32 C (6.5 min) at 22.88 C/min Sample 1-µL injection, PAH.1 to 5 ppm concentration range MSD Scan 45 4 u Samples = 2 2 Autotune EM + 2V Source = 23 C Quad = 15 C Transfer line = 28 C Results and Discussion The test sample with 16 polynuclear aromatic hydrocarbons (PAHs) was chosen for this example as it covers a wide range of physical properties and provides several challenging separations. This requires the retention time precision to be as tight as possible to ensure correct identification of the peaks of interest, and relocking must be effective or peak identification will fail on the different column. Samples were injected in triplicate to measure the retention time precision of the locked method at different concentrations from.1 to 5 ppm. Table 2 shows the performance metrics for the retention time and also the correlation co-efficient for the analysis. The initial retention times in Table 2 were achieved at a head pressure of 3.761 psig. This setpoint was determined from the retention time calibration and relocking process to be appropriate to achieve the target retention time of 11.112 min for the locking compound fluoranthene. Table 3 shows the calibration data from the RTL runs. Table 3. RTL Data Pressure Retention Deviation Run (psi)* time (mins) (seconds) RTLOCK1.D 3.1 11.212 6.18 RTLOCK2.D 3.38 11.166 3.264 RTLOCK3.D 3.76 11.112. RTLOCK4.D 4.14 11.7 2.58 RTLOCK5.D 4.51 11.2 5.58 Maximum deviation 6.18 seconds Correlation co-efficient.999 * Even though pressure setpoints used for RTL calibration need only be to be to two decimal places, the ability to precisely set locking pressures based on the calibration curve requires setability to the third decimal place. Figure 1 shows the calibration curves corresponding to the linearity metrics summarized in Table 2. Table 2. Retention Precision of Low-Pressure RTL Average retention RSD Calibration RT before Relocked ΔRT when time (%) linearity (r 2 ) relocking RT relocked Naphthalene 5.884.55.997 5.89 5.876.8 Acenapthylene 7.753.45.997 7.676 7.746.7 Acenaphthene 7.968.44.997 7.893 7.959.9 Fluorene 8.562.48.997 8.486 8.556.6 Phenanthrene 9.72.89.997 9.621 9.692.1 Anthracene 9.752.44.996 9.671 9.746.6 Fluoranthene 11.119.31.996 11.41 11.116.3 Pyrene 11.388.9.996 11.38 11.383.5 Chrysene 12.88.94.996 12.719 12.799.9 Benz[a]anthracene 12.855.46.997 12.774 12.849.6 Benz[b]fluoranthene 14.285.45.995 14.168 14.277.8 Benz[k]fluoranthene 14.314.69.995 14.21 14.36.8 Benz[a]pyrene 14.815.35.996 14.686 14.87.8 Indeno[1,2,3-cd]pyrene 17.88.51.996 16.89 17.74.14 Dibenz[a,h]anthracene 17.98.66.995 16.93 17.82.16 Benzo[ghi]perylene 17.725.89.997 17.58 17.79.16 2
Response 45 35 25 15 5 Naphthalene Acenapthylene Acenaphthene Fluorene Phenanthrene Anthracene Fluoranthene Pyrene Chrysene Benz[a]anthracene Benz[b]fluoranthene Benz[k]fluoranthene Benz[a]pyrene Indeno[1,2,3-cd]pyrene Dibenz[a,h]anthracene Benzo[ghi]perylene 1 2 3 4 5 6 Concentration ppb Figure 1. GCMS calibration curves for 16 PAHs using 32-µm id column with low-head pressure. Figure 2 presents overlayed chromatograms for the replicate sample injections, showing the excellent precision of the replicate injections summarized in Table 2. 26 22 18 TIC: 18224.D\data.ms TIC: 18123.D\data.ms TIC: 18325.D\data.ms 14 6 2 9.6 9.8 1. 1.2 1.4 1.6 1.8 11. 11.2 11.4 38 34 26 22 18 14 6 2 Figure 2. 4. 6. 8. 1. 12. 14. 16. 18. TIC: 18224.D\data.ms TIC: 18123.D\data.ms TIC: 18325.D\data.ms Overlay of three replicate injections of 2 ppm standard prior to column maintenance and relocking, including a zoomed area of the four peaks from phenanthrene to pyrene. 3
A 45-cm length of column was removed from the column to simulate typical maintenance that may be performed on a column during routine use. The method was then relocked and the sample re-run to check the efficacy of relocking at low pressure. The relocked method had a resulting pressure of 3.168 psig. The change in locking pressure can, over time, provide guidance as to the extent at which column trimming can be undertaken without the need for full re-locking of the method. Figure 3 shows an overlay of the before and after column trimming and the extent of the retention time change. Figure 4 presents an overlay of chromatograms, one of the originals and one after column trimming and relocking. The last two columns of Table 2 compare the relocked retention times of target compounds to the originals. 1e+7 9 8 7 6 5 TIC: RELOCK1.D\data.ms TIC: 19227.D\data.ms 9.4 9.6 9.8 1. 1.2 1.4 1.6 1.8 11. 11.2 11.4 1e+7 TIC: RELOCK1.D\data.ms TIC: 19227.D\data.ms 9 8 7 6 5 4. 6. 8. 1. 12. 14. 16. 18. Figure 3. Overlay of injection before and after column maintenance showing the extent of retention time variation, including a zoomed area of the four peaks from phenanthrene to pyrene 4
1e+7 9 8 7 6 5 TIC: 19126.D\data.ms TIC: RELOCKCHECKA.D\data.ms 9.6 9.8 1. 1.2 1.4 1.6 1.8 11. 11.2 11.4 1e+7 TIC: 19126.D\data.ms TIC: RELOCKCHECKA.D\data.ms 9 8 7 6 5 4. 6. 8. 1. 12. 14. 16. 18. Figure 4. Overlay of injections before and after column maintenance and relocking, including a zoomed area of the four peaks from phenanthrene to pyrene. Conclusions This application demonstrates the ability of the 789 GC system to perform RTL at low pressures (sub 5 psi), such as those experienced when using a 32-µm column in a GC-MS system. The average retention time variation before and after column maintenance for a 16-PAH mixture is less than.5 sec, providing high confidence in peak assignments, even with critical separations. For More Information For more information on our products and services, visit our Web site at www.agilent.com/chem. 5
www.agilent.com/chem Agilent shall not be liable for errors contained herein or for incidental or consequential damages in connection with the furnishing, performance, or use of this material. Information, descriptions, and specifications in this publication are subject to change without notice. Agilent Technologies, Inc. 28 Printed in the USA July 11, 28 5989-8366EN